1. Chapter 18.1: Viruses

2. 1. Viruses are composed of nucleic acids enclosed in a protein coat and are smaller than the smallest bacterium.

3. 2. Most biologists consider viruses to be nonliving because they don’t exhibit all the criteria for life.

4. 3. All viruses can do is replicate--make copies of themselves—and they can’t even do that without the help of living cells.

5. 4. A cell in which a virus replicates is called the host cell.

6. 5. A virus that infects a bacterium is called a bacteriophage, or phage for short.

7. 6. A virus has an inner core of nucleic acid, either RNA or DNA, and an outer protein coat called a capsid.

8. 7. The core of nucleic acid contains a virus’s genetic material.

9. 8. Viral nucleic acid is either DNA or RNA and contains instructions for making copies of the virus.

10. 9. The arrangement of proteins in the capsid of a virus determines the virus’s shape.

11. 10. Before a virus can replicate, it must enter a host cell.

12. 11. Before it can enter, it must first recognize and attach to a receptor site on the plasma membrane of the host cell.

13. 12. Each virus has a specifically shaped attachment protein.

14. 13. In general, viruses are species specific, and some also are cell- type specific.

15. 14. Name three different viruses.

16. 15. Once attached to the plasma membrane of the host cell, the virus enters the cell and takes over its metabolism.

17. 16. Only then can the virus replicate.

18. 17. Once inside the host cell, the viral genes alter the host cell to make new viruses.

19. 18. The host cell uses its own enzymes, raw materials, and energy to make copies of viral genes that along with viral proteins are assembled into new viruses, which burst from the host cell, killing it.

20. 19. The new viruses can then infect and kill other host cells.

21. 20. This process is called a lytic cycle.

22. 21.

23. 22. Not all viruses kill the cells they infect. Some viruses go through a lysogenic cycle, a replication cycle in which the virus’s nucleic acid is integrated into the host cell’s chromosome.

24. 23. Every time the host cell reproduces, the provirus is replicated along with the host cell’s chromosome.

25. 24. At any time, the provirus can be activated and enter a lytic cycle.

26. 25. Then the virus replicates and kills the host cell.

27. 26. Many viruses are RNA viruses— RNA being their only nucleic acid.

28. 27. The RNA virus with the most complex replication cycle is the retrovirus.

29. 28. Once inside the host cell, the retrovirus makes DNA from its RNA

30. 29. Name one retrovirus.

31. 30. Some viruses have been linked to certain cancers in humans and animals.

32. 31. These viruses disrupt the normal growth and division of cells in a host, causing abnormal growth and creating tumors.